Foliar and microscopic observations of bean leaves exposed to hydrogen chloride gas

Pinto bean plants, eight days from seeding, were exposed for 20 min to different concentrations of gaseous HCl. Macroscopic injury to the primary leaves examined 24 hr later consisted of bifacial interveinal necrosis and glazing of the abaxial leaf surface. Only abaxial glazing was observed when less than 17.9 mg HCl m^-3 was used. At higher concentrations, necrotic lesions were also present. The frequency of injured primary leaves, the number of primary leaves with necrotic lesions, and the necrotic area per primary leaf increased with increased HCl concentrations in the range of 6.0-54.2 mg m^-3. At all HCl concentrations, the distribution of injury was variable. At the end of the exposure period and at various intervals throughout the subsequent 24 hr period, segments were taken from the primary leaves and examined by light microscopy. Alterations noted in the treated tissues were plasmolysis, cytoplasmic vesiculation, cell wall deformation, transient appearance of crystalline figures in chloroplasts, and accumulation of particulates in vacuoles. The glazed abaxial epidermis was produced by complete collapse of the epidermal cells. Tissues most sensitive to HCl gas were the two epidermal layers in contrast with previous reports. The mechanism for HCl gas phytotoxicity may initially be a surface phenomenon rather than depending upon entry through stomates.